The present invention relates to a web conveying roller, and a process for the manufacture of a roller used for conveying a web, so that dynamic air entrained between the roller surface and the web can be vented from the roller surface when the roller and web are in contact.
In many manufacturing operations a web is trained around a plurality of rollers as it is conveyed through a series of stations. Some of the rollers are drive rollers used for advancing the web and other rollers are simply idler rollers. Typically the web conveyance system is designed to avoid relative movement between the surface of the rollers and the web in order to avoid scratching or other damage to the web. This is especially important during the manufacture of sensitive materials, such as photographic film, paper and magnetic media where such relative movement can produce a surface defect in the final product. Thus it is desirable that the surface of the rollers be sufficiently smooth to avoid damage to the web by the rollers whether the rollers are drive rollers or idler rollers. At the same time, it is important that there be sufficient friction between the rollers and the web to enable the drive rollers to advance the web and to enable the idler rollers to be rotated by the web at the same surface velocity of the web when the web is in contact with the idler rollers.
In a web transport system using drive or idler rollers air can become entrained between the roller and the surface of the web. More specifically, movement of the web can force air into the entrance nip between the web and the surface of the roller, especially when the web is moving at high speeds. This boundary layer of air can cause at least partial separation between the surface of the web and the surface of the rollers. When this occurs, there is a change in the ability of the drive rollers to advance the web, and the web cannot efficiently rotate the idler rollers. As a result, relative movement can occur between the rollers and the web, causing quality defects in the web.
There have been attempts to solve the problems caused by the boundary layer of air between a roller and the web. One example is set forth in U.S. Pat. No. 4,426,757, issued Jan. 24, 1984 in the names of R. Hourticolon et al. The web guide roller disclosed in such patent has cavities on its outer surface which receive air carried with the moving web. More specifically, the cavities comprise a finely branched network of compression chambers that are arranged on the roller surface between plateau-like smoothly ground and polished areas which contact the web. Air in these chambers is compressed between the web and the roller. Air enters these chambers at the point where the web first contacts the roller, and the air is discharged from the chambers at the point where the web runs off the roller.
It is also known from U.S. Pat. No. 3,405,855, which issued Oct. 15, 1968 in the names of D. A. Daly et al, to provide a plurality of grooves in the surface of a roller to control the air bondary layer. The grooves as disclosed in this patent provide passages for the discharge of the air. These grooves are specially formed in the surface of the roller in a predetermined repeating pattern, e.g., by a cutting operation that is both expensive and time consuming. Also, the shape and size of the grooves must be carefully controlled to avoid leaving undesirable marks in the web transported around the roller. The undesirable marks may comprise impressions resulting from the web pressing against the edges of the grooves. Also the grooves can leave thermal defects caused, for example, by the portion of a web in contact with the roller surface drying differently than the portion of the web over the grooves. Moreover, the resulting patterned marks are more readily observed by the human eye than a random pattern of marks. Thus these marks are clearly undesirable, especially in photograhic products such as film or paper.